Method for in vitro oligonucleotide synthesis using H-phosphonates

ABSTRACT

A method is provided for the high fidelity, rapid and economical in vitro synthesis of oligonucleotides. Nucleoside H-phosphonates are condensed in seriatim using a dehydrating agent to produce a poly (nucleoside H-phosphonate). The product is oxidized to yield the desired oligonucleotide. A novel reagent is provided for multiple nucleoside additions in single cycles.

CROSS REFERENCE TO RELATED APPLICATION

This is a division of application Ser. No. 06/910,048, filed Sep. 19, 1986, now U.S. Pat. No. 4,959,463, which is a continuation-in-part of U.S. Ser. No. 787,280, filed Oct. 15, 1985, now abandoned.

This invention relates to the in vitro synthesis of oligonucleotides. In particular it relates to the preparation of deoxyoligonucleotides for use in recombinant host-vector systems and as intermediates in the preparation of labelled oligonucleotide probes for diagnostic assays.

Oligonucleotides are presently in vitro by one of two prevalent methods, the phosphoramidite method (for example see Adams et al., 1983, "J. Amer. Chem. Soc."105: 661 and Froehler et al., 1983 "Tetrahedron Lett." 24: 3171) or the phosphotriester route (German Offenlegungsshrift 2644432). Both methods, while efficient and widely accepted, employ large amounts of costly monomer. These methods are also complex. The phosphoramidite method requires an aqueous oxidation step after each condensation, while the triester method requires that the aborted subpopulation of oligonucleotides (those that have not had a monomer added during a cycle) be "capped" in a separate step in order to prevent further chain elongation of the aborted oligonucleotides in future cycles. It is an object of this invention to provide a method for the oligonucleotide synthesis that is more economical and reliable than the methods currently available.

It is known to prepare nucleoside phosphonates by condensation of nucleosides with phosphorous acid using aryl sulfonyl chloride (Sekine et al., 1979; "Tetrahedron Lett." 20: 1145), or carbodiimides (Schofield et al., 1961, "J. Am. Chem. Soc." page 2316). Hall et al. ([1957], "J. Chem. Soc." Nucleotides Part XLI, pp 3291-3296) further treated the nucleoside H-phosphonate to prepare the corresponding dinucleotide phosphonate using diphenylchlorophosphate followed by oxidation to produce the corresponding dinucleotide phosphate. Thus, notwithstanding knowledge in the art relating to the preparation of dinucleotide phosphate since 1957, and the intervening development of two complex and costly methodologies for the in vitro preparation of oligonucleotides, the Hall et al. disclosure has remained unexploited for the preparation of oligonucleotides.

Garegg et al., "Chemica Scripta" 25: 280-282 (1985) found that 5'-O-dimethoxytritylthymidine 3'-hydrogenphosphonate reacts rapidly with 3'-O-benzoylthymidine in the presence of an activator such as diphenylphosphorochloridate (also known as diphenylchlorophosphate), 2,4,6-triisopropylbenzenesulfonyl chloride or 2,4,6-triisopropylbenzenesulfonyl tetrazolide to form, in quantitative yield, a phosphonate diester bond. According to Garegg et al., the stability of the 3'-phosphonates when compared to the corresponding 3'-phosphoramidites, together with their comparable reactivity to phosphoramidites on activation, made them suitable as starting materials for oligonucleotide synthesis. The Garegg activating agents have been found by the inventors herein to be unsuitable for practical oligonucleotide synthesis because of the poor yields so obtained, a discovery that may help to explain the three decades that have passed without the art having harnessed phosphonate chemistry to the objective of oligonucleotide synthesis.

The synthesis of DNA containing internucleotide phosphate analogues is becoming a field of great interest. Miller and Ts'o have shown that methyl phosphonate analogues of DNA readily pass through cell membranes and inhibit protein synthesis, presumably by interfering with mRNA translation (Blake et al., Biochemistry 24, 6139 (1985), Smith et al., Proc. Natl. Acad. Sci. 83, 2787 (1986)). Phosphoramidate analogues of dinucleotides are known to bind to complementary polynucleotides and can be used for the attachment of various ligands to DNA (Letsinger et al., Nucl. Acids Res. 14, 3487 (1986)). Oxidation of dinucleoside H-phosphonates in the presence of amines leads to the corresponding dinucleotide phosphoramidates (Nemer et al., Tetrahedron Lett. 21, 4149 (1980), Atherton et al., J. Chem. Soc. 660 (1945)) in high yield. Methods are needed for the facile preparation of phosphoramidate, thiophosphate and phosphate triester analogues of oligonucleotides.

SUMMARY

The objectives herein are accomplished by a method for synthesizing a polynucleotide which comprises (a) providing a nucleoside bound to a carrier, (b) condensing a blocked nucleoside H-phosphonate with the 5' hydroxyl of the carrier-bound nucleoside in the presence of a dehydrating agent, (c) removing the 5' blocking group of the nucleoside H-phosphonate of step b), (d) sequentially repeating steps b) and c) using selected nucleoside H-phosphonates until a polynucleotide H-phosphonate having greater than two nucleotides has been obtained, (e) oxidizing the polynucleotide H-phosphonate to form the corresponding polynucleotide, and (f) separating the polynucleotide from the carrier. The polynucleoside H-phosphonates are useful as starting materials for the novel synthesis of the phosphoramidate, thiophosphate and phosphate triester oligonucleotides.

DETAILED DESCRIPTION

The starting materials for the practice of the inventive method comprise a carrier-bound nucleoside and blocked nucleoside H-phosphonate monomers. The 5' hydroxyl of the carrier bound nucleoside is not blocked. The nucleosides employed herein are the monomers of RNA or DNA or their derivatives. Exemplary ribose-containing nucleosides includes adenosine, guanosine, inosine, cytidine, thymine ribonucleoside, and uridine. Deoxyribose-containing nucleosides include deoxyadenosine, deoxycytidine, deoxyguanosine, and thymidine. The first group of nucleosides are used in synthesizing mRNA or tRNA, the second group in preparing DNA.

The preferred blocked nucleoside H-phosphonate monomers are analogues of the nucleotides constituting nucleic acids. The blocking groups generally are triphenylmethyl ethers of the ribose or deoxyribose hydroxyl substituents. Their function is to restrict the condensation reaction to the 5' hydroxyl of the carrier-bound nucleoside or oligonucleotide. The preferred starting phosphonates for the preparation of oligodeoxynucleotides and their corresponding nucleosides, designated parenthetically, are 5'dimethoxytrityl-3'-thymidine H-phosphonate (thymidine), 5' dimethoxytrityl-3'-[-N⁶ -benzoyldeoxyadenosine] H-phosphonate (deoxyadenosine), 5' dimethoxytrityl-3'-[N⁴ -benzoylcytidine] H-phosphonate (deoxycytidine), and 5' dimethoxytrityl-3'-[N² -isobutyl deoxyguanosine] H-phosphonate (deoxyguanosine). The starting materials for the preparation of oligoribonucleotides are similar, for example for adenosine, 5' dimethoxytrityl-2'-dimethyl t-butyl silyl-[N⁶ -benzoyladenosine]-3' H-phosphonate. The starting phosphonates are prepared by methods known per se. For example, see Sekine et al., "Tetr. Lett." 16, 1711 (1973). Another method for preparing the starting phosphonates (which is preferred) employs tris (1, 2, 4-triazoyl) phosphite (TTP). TTP is conveniently generated in situ from phosphorus trichloride (PCl₃), 1, 2, 4-triazole and N-methyl morpholine in anhydrous methylene chloride (CH₂ Cl₂).

Carriers are selected that are insoluble in the reagents used for the oligonucleotide synthesis, this being for convenience in recovering the polynucleotide after each condensation cycle. The carrier also is sufficiently porous to permit the facile entry of reactants and elution of product and reactants. A preferred carrier is controlled pore glass, although silica gel or polystyrene also are acceptable. Carrier-bound nucleosides are prepared by known procedures, for example Chow et al., "Nucl. Acids Res." 9: 2807 (1981).

Nucleoside H-phosphonates exist as an anion having a positively charged counterion. Triethyl ammonium (TEAH⁺) or 1,8 Diazabicyclo [5.4.0] undec-7ene-onium (DBUH⁺) are preferred counterions. The DBUH salts of phosphonates have increased stability in anhydrous solvents.

The carrier bound nucleoside and the nucleoside 3' H-phosphonates are selected in seriatim so as to correspond to the predetermined sequence of the oligonucleotide to be synthesized, with the 3' nucleoside being carrier-bound. Oligonucleotide chain elongation proceeds in conformance with this predetermined sequence in a series of condensations, each one of which results in the addition of another nucleoside to the oligomer.

In the first of these condensations, the carrier-bound nucleoside is condensed with the second nucleoside in the oligonucleotide. The key reagent in this reaction as well as in the subsequent condensations is the dehydrating agent. Suitable dehydrating agents generally fall within the classes of phosphorylating agents or acylating agents and include isobutyl chloroformate, diphenyl chlorophosphate, organic acid anhydrides such as acetic anhydride, isobutyric anhydride and trimethylacetic anhydride, and organic acid halides such as pivaloyl chloride, pivaloyl bromide and benzoyl chloride. Diphenylchlorophosphate, carbodiimides, aryl sulfonyl chlorides and aryl sulfonyl tetrazolides in our experience result in extremely poor yields. The property of the acylating or phosphorylating agent that is most important for the purposes of preparing oligonucleotides is its ability to add a nucleoside to substantially all of the nascent carrier-bound chains at each cycle. If the additional nucleoside is not added to a subpopulation of chains, i.e. a cycle is aborted as to that subpopulation, then the resulting oligonucleotide generated by further addition to that subpopulation will be one base short. This will destroy the utility of the oligonucleotide as a component in DNA which is to be expressed in recombinant culture because the deletion will shift the DNA out of reading frame, and it is difficult to detect contaminating deletion mutants in long oligonucleotide products. Thus, when preparing oligonucleotides for use in molecular biology it is important to select a dehydrating agent that exhibits high fidelity in chain elongation, i.e., high yield addition at each cycle. Selecting such superior agents other than those specifically set forth herein is accomplished by the use of a screening assay.

The screening assay to be employed is described in more detail in Example 2. A nucleoside H-phosphonate(N) is condensed to a carrier-bound nucleoside through four cycles, the last one of which should yield N₅, then the polynucleotide H-phosphonates are separated from the carrier, converted to the oligonucleotides and separated on HPLC in order to determine the distribution of oligomers from N₂ to N₅. If the selected dehydrating agent produces an oligomer mixture having greater than about 80 mole % of N₅ as measured by spectrophotometric adsorption corrected for the extinction coefficients of the oligomers, and usually greater than about 90%, then the agent will be considered preferred for the purposes of this invention. Alternatively, the dehydrating agent that is selected is one which, after 49 cycles of step d) followed by steps e) and f), yields an oligonucleotide composition wherein the desired 50 nucleotide oligonucleotide in the composition is present in a greater molar amount than any other oligonucleotide species in the composition. The agents used by the art for the preparation of dinucleosides have been phosphorylating or sulfonylating agents. Agents of these classes used by the art are not capable of meeting the rigorous standards herein, particularly diphenyl chlorophosphate, 2,4,6-triisopropylbenzenesulfonyl chloride or 2,4,6-triisopropylbenzene sulfonyl tetrazolide.

The preferred activating agents are acylating agents. Particularly preferred are acylating agents having the structure ##STR1## wherein A is aryl or ##STR2## in which the R groups are inert substituents preferably selected from the group of the same or different normal or branched alkyl, aryl, heteroalkyl or heteroaryl, and X is halogen or a pseudo-halogen, e.g. a heterocyclic secondary amine or an active ester. R is preferably alkyl, X is preferably chloride or bromide. Pivaloyl chloride, wherein R is CH₃ and X is Cl, is the preferred acylating agent.

A representative heterocyclic amine X group is tetrazole. A typical example of this species is ##STR3## It is produced in accord with methods known per se wherein ##STR4## is reacted with tetrazole.

A representative active ester is ##STR5## produced by the reaction of ##STR6## with hydroxybenztriazole. Other suitable leaving groups will be apparent to the artisan.

The dehydrating agent typically is employed at a concentration of about 25 to 100 mM. Pivaloyl chloride preferably is present in a concentration of about 50 mM or 5 molar equivalents of pivaloyl chloride relative to the phosphonate monomer. The use of 10 equivalents reduced the yield of oligonucleotide, thus indicating that competitive acylation is yield limiting and that no extraneous capping step is required. The pivaloyl chloride reacts with the small proportion of aborted chains to produce a pivolate ester that is incapable of participating in further condensations. Thus, incomplete chains are capped without any need to introduce a separate step or additional reagents.

The quantity of phosphonate required in the condensation generally ranges about from 5 mM to 20 mM, ordinarily about 10 mM (.sup.˜ 5 eq. relative to the 5'-OH component). This is considerably less than the amount of monomer employed in known oligonucleotide synthetic processes, which typically require about 50 mM.

The solvent for the condensation reaction is an anhydrous organic solvent, preferably anhydrous pyridine/acetonitrile in volume proportions of 1:1.

The reaction generally is conducted by mixing dehydrating agent and selected phosphonate solutions and then contacting the mixture with the carrier-bound oligonucleotide chain. Typically, the carrier is particulate and packed into a column over which the solution of monomer and dehydrating agent then is passe. The contact time generally is about from 0.5 to 5 minutes, with about 1.5 minutes being preferred. The reaction temperature is about from 10° C. to 30° C., preferably 20° C.

The solution containing the residual dehydrating agent and nucleoside H-phosphonate is removed from the carrier after each cycle by washing with an organic solvent such as acetonitrile. Thereafter, the protecting group is removed from the added nucleoside, preferably by treatment with a 2.5% vol/vol dichloroacetic acid/CH₂ Cl₂ solution, although 1% w/v trichloroacetic acid/CH₂ Cl₂ or ZnBr-saturated nitromethane also are useful. Other deprotection procedures suitable for other known protecting groups will be apparent to the ordinary artisan. Then the carrier preferably is washed with anhydrous pyridine/acetonitrile (l/l, vol/vol) and the condensation reaction is repeated in as many additional cycles as are required to prepare the predetermined oligonucleotide.

After the required number of synthetic cycles the polynucleotide H-phosphonate is oxidized to the nucleic acid. In order to prepare oligonucleotides having phosphoramidate internucleotide linkages, the oxidation is conducted in the presence of a primary or secondary amine and, preferably, carbon tetrachloride. The preferred oxidizing agent is aqueous iodine to generate phosphate diesters into nucleotide linkages. Other representative oxidizing agents include N-chlorosuccinimide, N-bromosuccinimide, or salts of periodic acid. Thereafter, the oligonucleotide is separated from the carrier, in the preferred instance by incubation with concentrated ammonium hydroxide followed by filtration to remove the residual silica gel and evaporation of the oligonucleotide-containing solution. If desired the oligonucleotide then is purified by HPLC, polyacrylamide gel electrophoresis or other conventional techniques and the fidelity of the product confirmed by nucleic acid sequence analysis.

While the foregoing discussion has revolved around the consecutive addition of mononucleoside phosphonates it will be understood that more than one nucleoside can be added in a given cycle by using poly (nucleoside phosphonates), generally di- or trinucleoside phosphonates. These reagents are readily prepared by condensing a soluble nucleoside (which may or may not be bound to a carrier) to a nucleoside phosphonate otherwise as described above for chain initiation. Such dinucleoside diphosphonates are then employed in place of the mononucleoside phosphonates, or to prepare other poly(nucleoside phosphonates). These novel intermediates have the structure ##STR7## wherein Y is a counterion such as DBUH⁺, n is 1 or greater than 1, ordinarily 2, and M is a nucleoside. Ordinarily the nucleoside will be protected. An example is 5' Dimethoxytrityl-3'-thymidyl-5'-H-phosphonate-3'-thymidine-H-phosphonate. These agents should be purified and stored as anhydrous solids.

The following examples are illustrative of the invention but are not to be considered as limitations on its scope, which is defined by the appended claims.

EXAMPLE 1 Preparation of Eicosathymidylic Acid (T20) and Tetracontathymidylic Acid (T40)

The synthetic utility of 5'-dimethoxytrityl-3'-thymidine-H-phosphonate 1 was demonstrated by the synthesis of T₂₀ and T₄₀ on silica gel. 5'-Dimethoxytrityl-3'-thymidine H-phosphonate was prepared as previously described and characterized by ³¹ P NMR (δ-0.27 ppm, J(P-H)=605 Hz) (³¹ P NMR spectra were obtained in an anhydrous Pyr/CH₃ CN (1/1) solution and chemical shifts are reported relative to 5 percent phosphoric acid/D₂ O (external standard)). The reaction scheme (scheme 1) was as follows, wherein DMT is dimethoxytrityl, pyr is pyridine, DCA is dichloroacetic acid and R is succinyl silica. ##STR8## The polymer support (silica gel) was derivatized to 25 μmole 3'-succinyl thymidine per gram of silica gel as previously described (Chow et al., Id). The synthetic cycle consisted of a condensation reaction of 2 with 10 mM 1 (.sup.˜ 5 eq. relative to 5'-OH component 2) and 50 mM pivaloyl chloride at 20° C. in anhydrous pyridine/acetonitrile (1/1) for 5 min., followed by dimethoxytrityl deprotection of 3 using 2.5% DCA/CH₂ Cl₂ (2 min). After the required number of synthetic cycles, the polythymidine H-phosphonate product 3 was oxidized to polythymidylic acid 4 with 0.2M I₂ in tetrahydrofuran/Pyr/H₂ O (90/5/5) (5 min). The product 4 was removed from the solid support (conc. NH₄ OH/5 hr./55° C.) and evaporated. HPLC analysis of the crude product T₂₀ demonstrated that the product peak coeluted with a T₂₀ standard prepared by the phosphoramidite method. The T₂₀ and T₄₀ crude products also were evaluated by polyacrylamide gel electrophoresis and visualized by U.V. shadowing. The analytical data clearly demonstrated the high yield synthesis of both T₂₀ and T₄₀. T₄₀ was further evaluated by 5'-end labeling with γ-³² P-ATP using polynucleotide T4 kinase followed by complete degradation with snake venom phosphodiesterase. By all criteria, the synthetic T₄₀ was comparable to a standard T₄₀.

EXAMPLE 2 Identification of Preferred Dehydrating Agent

The method of Example 1 was repeated except that parallel cycles were run with diphenyl chlorophosphate (DPCP) as the dehydrating agent and the reaction was only continued through 4 cycles to pentathymidylic acid (T₅). The crude column eluate was subjected to HPLC analysis to determine the percentage of oligonucleotides representing premature chain terminations. The HPLC resin was the Zorbax-NH₂ brand. The flow rate was 2.5 ml/min. of a linear gradient of 25 mM to 250 mM KH₂ PO₄ in 15% vol/vol CH₃ CN (pH 5.5). Fractions were collected over 15 min. and adsorbance measured at 254 nm. The heights of the single, substantially symmetrical peaks for each of T₂, T₃, T₄ and T₅ were taken from the HPLC charts and converted into percentages of total polythymidine in each crude eluate, correcting for the coefficient of extinction for each oligomer (by multiplying the peak values by 2.5, 1.75, 1.25, and 1 respectively). The results are shown in the following table.

    ______________________________________                                                     Percent oligomer                                                   oligomer      DPDC    Pivaloyl chloride                                        ______________________________________                                         T2            25      2.5                                                      T3            23      3.5                                                      T4            44      5                                                        T5             8      89                                                       ______________________________________                                    

These results demonstrate that pivaloyl chloride is a preferred dehydrating agent, producing only about 11% of prematurely terminated oligomers through 4 cycles.

The yield of T5 using DPDC was only about 10% of that obtained by the use of pivaloyl chloride. Aryl sulfonyl chlorides and carbodiimides produced no detectable T5. Thus the method of this invention using the preferred acylating agents unexpectedly results in greater yields and reduces premature chain termination.

EXAMPLE 3 Preparation of Nucleoside H-Phosphonate

The following scheme was used to prepare deoxynucleosides for use herein. ##STR9##

To a stirred solution of PCl₃ (75 mmole) and N-methyl morpholine (750 mmole) in 750 ml anhydrous CH₂ Cl₂ was added 1,2,4-triazole (250 mmole) at room temperature. After 30 min. the reaction mixture was cooled to 0° C. and 5'-Dimethoxytrityl thymidine (Ia, 15 mmole, dried by co-evaporation from CH₃ CN) in 200 ml anhydrous CH₂ Cl₂ was added dropwise over 20 min., stirred for 10 min., poured into 600 ml of 1.0M aqueous triethylammonium bicarbonate (TEAB, pH 8.5), shaken and separated. The aqueous phase was extracted with 200 ml CH₂ Cl₂ and the combined organic phase was dried over Na₂ SO₄ and evaporated to a foam. Silica gel column chromatography (2 percent Et₃ N/CH₂ Cl₂ → 2 percent Et₃ N/10 percent MeOH/CH₂ CL₂) followed by TEAB extraction ad evaporation yielded 5'-dimethoxytrityl-3 '-thymidine H-phosphonate (1a) in 90 percent yield. All other deoxynucleoside H-phosphonates were prepared by the same procedure. Table 1 reports isolated yields and ³¹ P NMR data:

                  TABLE 1                                                          ______________________________________                                                      .sup.31 P NMR.sup.1)                                                             Chemical            Yield.sup.2                                 Nucleoside H-phosphonate                                                                      Shift      J(P-H)   (Percent)                                   ______________________________________                                         thymidine (1a) -0.3 ppm   605 Hz   90                                          deoxyadenosine (1b)                                                                           -0.3 ppm   600 Hz   85                                          deoxycytidine (1c)                                                                            -0.3 ppm   603 Hz   86                                          deoxyguanosine (1d)                                                                           -0.6 ppm   605 Hz   77                                          ______________________________________                                          .sup.1).sup.31 P NMR spectra were obtained in an anhydrous Pyr/CH.sub.3 C      (1/1) solution and chemical shifts are reported relative to 5 percent          phosphoric acid/D.sub.2 O (external standard).                                 .sup.2 Isolated yield based upon protected deoxynucleoside (Ia-d).       

EXAMPLE 4 Synthesis of Deoxyoligonucleotides

This currently represents the best mode for practicing the invention. Syntheses were performed with a Biosearch Model 8600 DNA synthesizer. Trimethylacetyl chloride (pivaloyl chloride) was distilled at atmospheric pressure and stored under Argon (Ar). Pyridine (Pyr) was distilled from p-toluenesulfonyl chloride, then freshly distilled from calcium hydride. Acetonitrile (CH₃ CN) was dried over activated 3 Angstrom molecular sieves. Deoxy-nucleoside H-phosphonates (1a-d) were dried by co-evaporation from anhydrous CH₃ CN and reconstituted in anhydrous Pyr/CH₃ CN (1/1). Syntheses were performed on control pore glass (0.1 μmole scale) using the following protocol.

Synthetic Cycle (Scheme 1, Example 1)

1) Wash -- anhydrous CH₃ CN (45 sec).

2) Deblock -- 2.5 percent dichloroacetic acid (DCA)/CH₂ Cl₂ (1 min.).

3) Wash -- anhydrous Pyr/CH₃ CN (45 sec).

4) Couple -- 10 mM deoxynucleoside H-phosphonate (1a-d), 50 mM pivaloyl chloride in anhydrous Pyr/CH₃ CN (1/1 by vol) (1.5 min).

5) Repeat steps 1-4 until the nucleotide sequence is complete.

6) Deblock -- 2.5 percent DCA/CH₂ Cl₂ (1 min).

7) Oxidize H-phosphonate DNA with: 1) 0.1M I₂ in Pyr/NMI/H₂ O/THF (5/1/5/90 by vol) (2.5 min), 2) 0.1M I₂ in Et₃ N/H₂ O/THF (5/5/90 by vol) (2.5 min). The DNA was removed from the polymer, deprotected (conc. NH₄ OH, 55° C., 5 hrs.) and evaporated.

Deoxynucleoside H-phosphonates (1a-d) have been used for the complete synthesis of deoxyoligonucleotides (DNA) up to 107 bases in length, although this is not an upper limit on synthetic capability. The synthetic protocol described above uses a minimal amount of deoxynucleoside H-phosphonate (2.5 mg/coupling) in a simplified and rapid procedure (4 min/cycle). The internucleoside phosphorus is protected from unwanted side reactions by its oxidation state and the native phosphate diester is generated after synthesis by aqueous I₂ oxidation.

Oxidation of a polymer bound polynucleoside H-phosphonate (3) with 0.1M I₂ in a Pyr/H₂ O/THF (5/5/90) solution has been inadequate for consistent results. The aqueous I₂ oxidation of dialkyl H-phosphonates is subject to general base catalysis (Lewis, E. S. and Spears, L. G., J. Amer. Chem. Soc., 107, 3918 (1985)) and we have determined that addition of N-methyl imidazole (NMI) or stronger bases (i.e. N-methyl morpholine or triethylamine (Et₃ N) increases the rate of oxidation and is essential for the oxidation of long deoxyoligonucleoside H-phosphonates (3, >40 bases). Dialkyl H-phosphonates are susceptible to alkaline hydrolysis (Kume, A., Fujii, M., Sekine, M. and Hata, T., J. Org. Chem., 49, 2139 [1984]) and we have observed that the aqueous oxidation of polynucleoside H-phosphates (3) using Et₃ N is complicated by competitive hydrolysis. The I₂ oxidation of long deoxyoligonucleoside H-phosphonates (3), >40 bases) first with a weakly basic solution (Pyr/NMI) followed by oxidation with a strongly basic solution (Et₃ N) gives the highest yield of product deoxyoligonucleotide (4). Additionally we have found that an aqueous I₂ solution disproportionates to IO₃ ⁻ and I⁻ (Lewis, Id.) and an alkaline IO₃ ⁻ solution will not oxidize a dinucleoside H-phosphonate (3) (Brown, D. M. and Hammond, P. R., J. Chem. Soc., 4229 [1960]). Therefore it is necessary to prepare separate solutions [A-0.2M I₂ /THF, B-Base/H₂ O/THF (1/1/8)] and mix them immediately prior to use for the consistent and rapid oxidation of deoxyoligonucleoside H-phosphonates (3).

Experiments were performed to evaluate the stability of the deoxyoligonucleoside H-phosphonate linkage (3) to the reagents used in deoxyoligonucleotide synthesis. A 24-mer was prepared and, prior to oxidation of the deoxynucleoside-H-phosphonate (3), aliquots of the solid support were treated for 4 hours with: 1) anhydrous Pyr/CH₃ CN (1/1), 2) anhydrous CH₃ CH, 3) 100 mM pivaloyl chloride in anhydrous Pyr/CH₃ CN (1/1) and 4) 2.5 percent DCA/CH₂ Cl₂, washed with CH₃ CN, oxidized, washed, deprotected (conc. NH₄ OH/5 hr./55° C.) and evaporated. After 5'-end labeling with γ-³² P-ATP using polynucleotide T4 kinase the samples were analyzed by gel electrophoresis and autoradiography (data not shown). Under these conditions no detectable degradation of the polynucleoside H-phosphonate (3) is observed, but addition of 1 percent H₂ O to CH₃ CN or the Pyr/CH₃ CN solution leads to detectable degradation in the 4 hr. time period. These results indicate that the H-phosphonate linkage serves as a phosphorus protecting group throughout synthesis.

It has been shown that 3' or 5'-phosphate triester stabilizes the glycosidic bond of deoxyadenosine to acid mediated depurination (Tanaka, T. and Letsinger, R. L., Nucl. Acids Res., 10, 3249 (1982)). The effect an internucleoside H-phosphonate linkage (3) would have on the rate of depurination was assessed. A polymer bound 3'-thymidine5'-deoxyadenosine dinucleoside H-phosphonate, protected with a N⁶ -benzoyl amide (3b,a) or N⁶ -diisobutyl formamidine (3e,a), was treated with 2.5 percent DCA/CH₂ Cl₂ for 24 hours and analyzed. No detectable difference in rate of depurination between the H-phosphonate diester (3) and the methyl phosphate triester was observed (data not shown). This experiment also confirmed the added stability of the N⁶ -amidine protected deoxyadenosine relative to the N⁶ -benzoyl amide protected deoxyadenosine.

Specific sequences of DNA were synthesized using the described protocol and the crude product characterized.

                                      TABLE                                        __________________________________________________________________________     Sequences of synthesized deoxyoligonucleotides                                 __________________________________________________________________________       78a                                                                              ##STR10##                                                                   78b                                                                              3'-GAGCATTTTTCCCATAGCTGTTACTTTCGTTAAAAGCATGACTTTCCAAGTGACCT                     ##STR11##                                                                     GCTTAAGACTAACTTACTCTAG-5'                                                    99                                                                               5'-AGTAGCAAGCTTGAGGTGTGGCAGGCTTGAGATCTGGCCATACACTTGAGTGACAATGAC                ATCCACTTTGCCTTTCTCTCCACAGGTGTCCACTCCCAG-3'                                  107a                                                                              5'-AATTCATGAAAAAGAATATCGCATTTCTTCTTGCATC TATGTTCGTTTTTTCTATTGCTA            107b                                                                              3'-GTACTTTTTCTTATAGCGTAAAGAAGAACGTAGATACAAGCAAAAAAGATAACGAT                    CAAACGCGTATGCAGACTCATGGATGGAGGAAGTTATTAAATTATGC-3'                             GTTTGCGCATACGTCTGAGTACCTACCTCCTTCAATAATTTAATACGCCGG-5'                      __________________________________________________________________________

The deoxyoligonucleotides 78a, 78b and 99 were also synthesized with methoxy diisopropylamino phosphoramidites and are included in the Table for comparison. The amidite syntheses were carried out using a standard protocol except that a 30 second aqueous wash was incorporated prior to oxidation to hydrolyze possible phosphite-heterocyclic base adducts. The data presented in the Table clearly demonstrate the high yield synthesis of product deoxyoligonucleotides using deoxynucleoside H-phosphonate intermediates. At this time the sequence fidelity of H-phosphonate DNA is lower than that produced by phosphoramidites; however, this disadvantage is offset by a simpler synthetic cycle and significantly lower concentration of deoxynucleoside H-phosphonate required for synthesis.

EXAMPLE 5

The dinucleotide phosphoramidates 2e and 2f (Scheme 3) were initially prepared by oxidation of the polymer bound dinucleoside H-phosphonate 1 ##STR12## with 0.05M I₂ in a solution of the corresponding amine (10 percent) and tetrahydrofuran (THF) for 5 min. The product amidates were removed from the solid support (50 percent conc. ammonium hydroxide (NH₄ OH/Dioxane, 8 hrs., r.t.) and evaporated. ³¹ P NMR spectra of both products show the presence of approximately equal amounts of diasteriormers (³¹ P NMR spectra were obtained in CDCl₃ and chemical shifts are reported relative to 5 percent phosphoric acid/D₂ O (ext. std.); 2e δ-9.1, -8.8 ppm; 2f δ -7.9, -7.6 ppm.). The oxidation of H-phosphonate diesters to phosphoramidates is complicated by competitive hydrolysis, generating the phosphate diester 2a. Carbon tetrachloride (CCl₄) oxidation of H-phosphonate diesters to phosphoramidates (Atherton et al., J. Chem. Soc. 660 (1945), Zwierzak, A., Synthesis 507 (1975)) offers the advantage of low water solubility into CCl₄. All dinucleotide phosphoramidates (2b-g) were prepared with a 10 percent solution of the corresponding amine in CCl₄ (5 min) (A saturated solution of ammonia or methyl amine in Dioxane/CCl₄ (1/4) was used to prepare the corresponding phosphoramidates). High performance liquid chromatography (HPLC) of the samples indicate a high yield of product phosphoramidate with very little competitive hydrolysis (<3 percent dithymidine phosphate diester, 2a).

The stability of the phosphoramidate linkage to conc. NH₄ OH was examined for each and the results indicate the phosphoramidates 2c-g are stable to the conditions necessary for removal of the common heterocyclic N-acyl protecting groups (55° C., 5 hrs). The phosphoramidate 2b rapidly decomposes under these conditions (t_(1/2) .sup.˜ 15 min) to a mixture of the 3' and 5' phosphoramic acid monoesters (Tomasz, J., Nucleosides and Nucleotides 2, 51 (1983)). The stability of the dinucleotide phosphoramidates (2b-g) to enzymatic degradation by exonucleases was examined using spleen phosphodiesterase and snake venom phosphodiesterase. Incubation of the dinucleotides (2a-g) with the corresponding enzyme/buffer solution at 37° C. for 1 hr led to complete degradation of the phosphate diester (2a) with no detectable degradation of the phosphoramidates 2b-g (assessed via reverse phase HPLC). The observed enzymatic stability of the dinucleotide phosphoramidate 2b is contrary to the claims of Ogilvie (Nemer et al., Tetrahedron Lett. 21, 4149 (1980)) but is in agreement with those reported by Letsinger (Letsinger et al., Nucl. Acids Res. 14, 3487 (1986)).

To further explore the scope of this oxidation procedure, undecathymidylic acid (T₁₁) was prepared containing from 1 to 9 phosphormorpholidate linkages. The synthesis of polythymidine H-phosphonate on controlled pore glass was carried out using the synthetic protocol described above. The strategy outlined in Scheme 4 demonstrates the synthesis of T₁₁ containing nine phosphormorpholidate linkages followed by one phosphate diester linkage. This strategy was used for all T₁₁ products containing 3' phosphoramidate linkages. ##STR13##

The products were removed from the solid support with conc. NH₄ OH (2 hr/r.t.) and evaporated. Polyacrylamide gel electrophoresis (PAGE) and U.V. shadowing indicated a major product band with each. The band was cut, crushed, eluted with H₂ O and isolated by reverse phase HPLC (C₁₈). An autoradiogram was derived from 5'-end labeling of these products with T4 polynucleotide kinase and γ-³² P ATP followed by PAGE (17 percent polyacrylamide/7M urea). The change in electrophoretic mobility of the different T₁₁ products was very pronounced, each additional morpholidate resulting in an increase in mass and a decrease in charge. It is interesting to note that the electrophoretic mobilities of the different T₁₁ products do not follow a strict mass/charge ratio dependence. The products containing many phosphoramidate linkages (high mass/charge ratio) have greater mobility than would be predicted based upon the mobilities of the T₁₁ products with fewer phosphoramidates. Treatment of each of these polythymidine phosphormorpholidates with 85 percent formic acid (95° C., 15 min) generated T₁₁ containing all diester linkages.

Nucleoside H-phosphonate diesters have been used as precursors to thiophosphate analogues (Fujii et al., Tetrahedron Lett. 26, 935 (1986)). This conversion is accomplished in high yield (>98 percent) directly from the polymer bound oligonucleoside H-phosphonate diester (1) by treatment with a solution of 0.1M S₈ in Triethylamine (TEA)/carbon disulfide (1/9, 5 min). In order to prepare radioisotopically labelled oligonucleotides having extremely high specific activity for use as hybridization probes, the S₈ that is employed will contain a proportion of ³⁵ S radioisotope.

Triesters of oxygen are prepared by reaction of oligonucleoside H-phosphonates (1) with a 10 percent solution of the corresponding alkanol (ROH) in N-methyl imidazole /TEA/CCl₄ (5/5/90). R is aryl, alkyl, heteroalkyl or heteroaryl generally having less than 10 carbon atoms, preferably MeOH or n-BuOH. This reaction is very susceptible to competitive hydrolysis and care must be taken to assure anhydrous conditions.

The primary and secondary amines b)-g) are to be considered merely representative. It is within the scope herein to employ substituents having the general formula ##STR14## wherein X and Y are the same or different and are selected from the group of hydrogen, alkyl, heteroalkyl, heteroaryl or aryl, or are taken together to form a cycloalkene, cycloalkane or heterocycle. The species in which X or Y are, or are conjugated to form heteroalkyl and heteroaryl substituents are preferred for use as in vitro hybridization probes, whereas the alkyl or aryl substituents are preferred for in vivo hybridization.

The heteroatom in heteroaryl ROH, X or Y is selected from the group of S, N or O, about from 1 to 5 of which are present in the heteroaryl group. Heteroaryl species that are particularly useful in in vitro diagnostics are those in which the heteroaryl group is fluorescent, for example, rare earth metal chelates for use in pulsed fluroescence, or other known substituents for use in polarized fluorescence hybridization assays. Such fluorescent labelled oligonucleotides are particularly useful in hybridization assays because it is possible to conduct the assays without a phase separation by detecting changes in fluorescence upon hybridization to the target DNA, e.g. changes in polarization, emission wavelength, absorption wavelength, emission intensity and the like. The method herein facilitates the preparation of oligonucleotides which are heavily substituted with such fluorescent species and accordingly exhibit high specific activity.

The results presented above demonstrate that polynucleoside H-phosphonates are valuable precursors to a variety of internucleotide phosphate analogues. Oxidation of a polynucleoside H-phosphonate in the presence of amines leads to the corresponding polynucleotide phosphoramidate in high yield. The H-phosphonate method of DNA synthesis is a simple, fast and reagent efficient procedure that can now be applied to the rapid synthesis of DNA containing internucleotide phosphate analogues. This method provides an easy and efficient route to a variety of internucleotide phosphate analogues of DNA containing the naturally occurring nucleotides. 

We claim:
 1. A method for preparing a polynucleotide phosphoramidate, said method comprising the steps of:oxidizing a carrier-bound polynucleoside H-phosphonate having more than two nucleosides with an oxidizing agent capable of oxidizing said carrier-bound polynucleoside H-phosphonate so as to yield carrier-bound polynucleoside phosphoramidate; and decoupling said polynucleoside phosphoramidate from said carrier so as to yield said polynucleotide phosphoramidate.
 2. A method as recited in claim 1 wherein a polymer carrier is bound to a 3'-hydroxyl of said polynucleoside H-phosphonate.
 3. A method as recited in claim 1 wherein said polynucleoside H-phosphonate is bound to a carrier of material selected from the group consisting of controlled pore glass, silica gel and polystyrene.
 4. A method as recited in claim 1 wherein said oxidizing agent is selected from the group consisting of iodine, N-chlorosuccinimide, N-bromosuccinimide, and salts of periodic acids.
 5. A method as recited in claim 1 wherein said carrier-bound polynucleoside H-phosphonate has at least 40 nucleosides and wherein said oxidizing step occurs:with a relatively weak basic solution containing an amine with 1-5 S, N, or O atoms and fewer than 10 carbon atoms; and then with a relatively strong basic solution containing an amine with 1-5 S, N, or O atoms and fewer than 10 carbon atoms.
 6. A method as recited in claim 1 wherein said decoupling step involves incubating a carrier to which said polynucleoside phosphoramidate is bound in concentrated ammonium hydroxide.
 7. A method as recited in claim 1 wherein said oxidizing step occurs in a solution containing a base selected from the group comprising ammonia, primary amines and secondary amines, said base containing 1-5 S, N, or O atoms and fewer than 10 carbon atoms.
 8. A method as recited in claim 7 wherein said amine includes an alkyl, an aryl, or an heteroaryl.
 9. A method as recited in claim 7 wherein said amine is a heterocyclic secondary amine.
 10. A method as recited in claim 7 wherein said solvent is selected from the group consisting of tetrahydrofuran and carbon tetrachloride.
 11. A method for preparing a polynucleotide phosphoramidate, said method comprising the steps of:oxidizing a carrier-bound polynucleoside H-phosphonate having more than two nucleosides in a solution of aqueous iodine and a base selected from the group consisting of NH₃, MeNH₂ and n-BuNH₂ in a solvent selected from the group consisting of tetrahydrofuran and carbon tetrachloride so as to yield carrier-bound polynucleoside phosphoramidate; and incubating a carrier to which said polynucleoside phosphoramidate is bound in concentrated ammonium hydroxide so as to yield said polynucleotide phosphoramidate. 